1. Field of the Invention
The present invention relates to a method for modifying the combining or splitting ratio of a multimode interference coupler, as well as a corresponding method for modifying the combining or splitting ratio of a circuit comprising several such couplers. It also relates to a coupler comprising a strained film.
2. Description of the Related Art
Multimode interference (MMI) couplers are well-known in the art, and are for instance used as combiners and splitters in integrated optical circuits. Light launched into an access port of an MMI coupler traverses the length of the MMI region in several modes towards one or several output ports at an opposite side of the coupler. Because of the self-imaging effect of the coupler, its geometry can be determined so as to give rise to a certain light intensity distribution at the output port or ports for certain wavelengths. In its function as a combiner, the coupler comprises at least two input ports. As a splitter, the coupler comprises at least two output ports. A frequently used configuration is a 2×2 MMI coupler, comprising two input ports and two output ports.
In a 2×2 MMI coupler, it is customary to designate the transmission from an input port to an output port that is situated on the same side of the coupler centerline as the bar state transmission, and to designate transmission to an output port situated on the other side of the coupler centerline as the cross state transmission.
In many applications, it is desirable for a 2×2 MMI coupler to have a combining or splitting ratio of 1. The splitting ratio of a 2×2 MMI coupler for light that is launched from one of the input ports can be defined as the power transmitted in the cross state divided by power transmitted in the bar state. For example, the splitting ratio is the cross output power divided by the bar output power. In the case of a splitter with two output ports, a splitting ratio of 1 thus implies a symmetric division of light power between the output ports. A ratio of 1 is desirable for example where the coupler is used in a Mach-Zehnder interferometer based modulator for laser communication, such as the one described in WO 2011/043718 A1.
However, once an MMI coupler has been manufactured, for instance as a part of an integrated optical circuit, it is difficult to correct any deficiencies of the coupler negatively affecting the resulting splitting or combining ratio.
Several methods have been proposed to control the splitting and/or combining ratio of MMI couplers:
U.S. Pat. No. 5,689,597 discloses a so-called butterfly-shaped or inverse butterfly shaped 2×2 port optical MMI coupler, which achieves asymmetric splitting or combining. The transmission properties of a butterfly MMI are a sensitive function of the MMI dimensions, where for example only a 0.2 um change in the maximum or minimum width of the MMI can have a very large influence on the combining or splitting ratio. Accordingly, it is difficult to alter the combining or splitting ratio of a butterfly MMI coupler after it has been fabricated by subsequent modifications of the MMI shape.
U.S. Pat. No. 6,571,038 describes different methods for achieving a tunable splitting ratio in a 2×2 port MMI coupler, such as by placing electrodes at the location of optical images in the MMI and injecting current there. This requires the application of electrodes on the coupler, along with the additional cost and energy consumption of such injection, as well as control circuitry and space requirements for the accommodation of such circuitry. Another variant according to this document is to illuminate locations of optical images in the MMI with a light beam. This requires the presence of an apparatus to generate and focus a light beam onto the said locations.
The article Trung-Thanh Le and Laurence W. Cahill, “The Design of SOI-MMI Couplers with Arbitrary Power Splitting Ratios Using Slotted Waveguide Structures”, LEOS Annual Meeting Conference Proceedings, 2009 proposes an asymmetric 2×2 splitter or combiner fabricated with a deeply etched slot along the length of the MMI, at or near the center of the MMI. The slot must have optically smooth sidewalls and a precisely controlled depth, and it should also have a flat bottom surface, in order to achieve the desired splitting ratio while keeping scattering loss and reflections to a minimum. This not only results in more complicated manufacturing, but also limits the possible material systems.